Proposals for the most radical re-drawing of the Welsh political map for almost 70 years have been criticised by Labour and nationalist politicians.

Under the plan the number of MPs representing Wales would be cut from 40 to 30 and changes to every Westminster constituency made.

Party workers will spend the next few days and weeks working out how the proposals would affect the political make-up of Wales and, ultimately, the House of Commons.

Because Wales has long been a Labour heartland, the changes are almost certain to affect the party disproportionately. A spokesman said: "These plans, as pushed by the Tory-Lib Dem government, have always been about reducing Wales' voice and influence at Westminster. Welsh Labour has opposed the plans on this basis and continues to do so."

Peter Hain, Labour's shadow Welsh secretary, called the cut in the number of MPs in Wales "diabolical".

The nationalists, Plaid Cymru, also criticised the reduction. "Plaid Cymru opposed the 25% cut in Welsh MPs at Westminster and this is still our position," said MP Jonathan Edwards.

He accepted the task of redrawing boundaries was going to be difficult.

But he added: "There are many proposals where geographical and community links have been ignored in order to reach a specific figure.

"Some of the proposed constituencies are not very practical – there are examples of two totally different communities being put together despite a large mountain separating them, as well as a number of proposals where two villages in the same community have been separated. We are also worried by the size of some of the proposed constituencies."

The changes are part of a UK-wide package to cut the number of MPs from 650 to 600. Under a law brought in last year every constituency must have an electorate between 72,810 and 80,473. Only one current Welsh constituency – Cardiff South and Penarth – falls within this range.

The Boundary Commission for Wales said the proposals were the "biggest single change to Wales" since the commission was created in 1944. Every constituency would be re-drawn and some changes would be "considerable" under the proposals, it said.

Changes to constituencies in the valleys such as Rhondda and Cynon, traditional Labour strongholds, will worry the party. But the Liberal Democrats could suffer in places like Cardiff, where significant changes are proposed.

There is surprise at some suggestions, such as a new Llanelli constituency taking in part of the city of Swansea, and some of north Cardiff being included in a new Caerphilly constituency.

Two constituencies, Gwynedd and South Powys, would be more than 1,500 square miles.

A North Wales Coast constituency would take in resorts like Abergele, Colwyn Bay, Conwy and Llandudno.

Ben Whitestone, secretary to the commission, said: "I would emphasise that these are initial proposals and I very much hope that members of the public will participate in the process of consultation which now begins."